Large motion color images, such as displayed in movie theaters, are formed by projection of an image onto a separate front or rear projection screen. The images are usually formed by projecting light through individual film frames illuminating a full screen, with frames succeeding one another at 20 to 30 times a second.
Film projection technology is approaching the limit of its technological evolution. Film requires the frequent changing of bulky reels of film. Film image quality degrades with repeated showings. Film has reached the economic limit of its resolution capabilities. The vibrancy of its colors falls short of the colors visualized in the real world. Film projection frequently encounters registration problems, where the image jumps around on the screen. This contrasts with recent technological advances in digital image storage, manipulation and transmission, wherein complete movies can be digitally recorded on optical disks and other digital storage media. Digital storage and distribution should be relatively inexpensive, and the image source does not degrade with repeated showings or duplications. Thus, movie projection utilizing a digital, electronically scanned (termed “video” herein) image source is a desirable alternative to film, assuming such an image can be projected with sufficient brightness, resolution, color balance, registration, and lack of motion artifacts to equal or exceed the capabilities of film. Laser projection is believed by many to offer the most promising solution to the problems inherent with film projection.
Laser video projectors have been used for the display of electronic images since about 1980, with the first projector built in England by the Dwight Cavendish Company. This projector used an Argon ion laser and a dye laser to produce standard television resolution images up to about ten feet across in a darkened room. The projector was almost as large as its image and was difficult to operate. The Dwight Cavendish laser projector, and indeed any laser projector, required the following basic components to make a video image: (a) lasers to supply the light that is sent to the screen to form the image; (b) a method of controlling the intensity of the laser light for each portion of the image, often called “modulation”; and (c) a method of distributing the modulated light across the screen surface, often called “scanning”.
An improved version of the Dwight Cavendish laser projector is described by Richard W. Pease in “An Overview of Technology for Large Wall Screen Projection using Lasers as a Light Source”, MITRE Technical Report, The MITRE Corporation (July 1990). The projector described in the MITRE publication utilized the following components corresponding to the laser source, modulator and scanner described above. The laser sources included argon ion lasers to produce 454 to 476 nm blue and 514 nm green, and Rhodamine 6G dye laser pumped with an argon ion laser to produce 610 nm red. The system used acousto-optic modulators between the laser sources and the scanning component for the laser beam of each color, with the modulated beams later combined with dichroic mirrors and deflected and focused onto the scanning component. The scanning section included a rotating polygon mirror and galvanometer-controlled frame mirror, as further described below. The rotating polygon mirror had 25 mirror facets, each of which deflected the modulated beam horizontally across a predetermined angle onto a mirror tilted vertically by a galvanometer across a predetermined angle through lenses onto the screen.
Several problems in particular limit the ability of current large screen projection technology to produce movie theater quality laser images. Because such laser projection systems typically used complicated lens and mirror systems to combine modulated colored beams into a composite beam to be scanned, and to scan and focus beams onto a screen, much of the power of the laser beams was sapped away, making laser projection images substantially less bright than that produced by film projection. Further, because certain wavelengths, especially blue, have been difficult to produce at adequate power levels with lasers, brightness and color balance have been inadequate for large screen video applications.
Currently known laser video systems typically use an Argon ion laser to make blue and green, and a ′flowing jet dye′ laser to make red. To provide some perspective, an Argon ion laser is 5 to 8 feet long, weighs 100 to 600 pounds and consumes from 15 to 75 kilowatts of electricity and two to five gallons per minute of cooling water. Such lasers cost $30,000 to $105,000 each. Several of these laser would be required for the optical power necessary to accomplish theater or large screen video display.
Known projection systems that used rotating polygon mirrors did not adequately address the problems of facet errors that would tend to slightly misdirect the beams, requiring complex optical or mirror array systems to compensate for the slight misdirections. The complex optics, facet error correction, and scanning systems also tended to cause color separation, causing the combined laser beams to diverge and degrade as they passed through these complicated optical paths.
Perhaps the most significant problem, however, with prior laser projections systems in comparison with film projection technology, is the lack of sufficient resolution. Attempts to increase resolution only exacerbate the other problems noted above. In order to effectively compete with or displace film projection, it is widely believed that laser projection systems must be capable of resolutions approaching 1900 by 1100 fully resolved pixels, or roughly the maximum resolution of the newly established HDTV standard of 1920×1080 p at 60 frames per second or more. HDTV is an acronym for “High Definition Television” and refers to an emerging standard for new home viewing and large image projection technology. While this standard addresses many formats, many believe that the highest resolution format, 1920×1080 pixels, especially in a “progressive” mode, defined herein (designated as “1920×1080 p”), represents more than sufficient resolution for theater displays. Standard television quality resolution, such as that projected by the Dwight Cavendish system, rarely exceeds 525 horizontal lines. For television to achieve this resolution, 525 horizontal lines of analog image data are scanned, roughly comparable to a digital pixel array of 525×525 pixels. Thus, television quality video would require, for a full frame rate of 30 frames per second, that a polygon mirror scan more than 945,000 lines per minute. For a 25 facet polygon mirror such as used in the Dwight Cavendish system, writing one line with each facet would require a rotation of more than 37,500 rpm. Because of centrifugal force limitations, rotational speeds this high limit the feasible size and/or number of the facets, thereby limiting resolution.
Moreover, the problems inherent in polygon mirrors frequently used in prior laser projection systems such as the Dwight Cavendish system are exacerbated when attempting to scan 1920×1080 HDTV quality video or better resolution video. The threefold or greater horizontal resolution mandates a larger facet size, yet the increased number of lines per frame mandates either an increase in the number of facets or substantially increased polygon mirror rotational speeds. For example, a 25 facet polygon mirror would have to have facets more than three times greater in width resulting in a polygon more than three times greater in diameter. For HDTV 1920×1080 p resolution at a full frame rate of at least 60 frames per second, this polygon of much larger facet widths would have to scan more than 3.8 million lines per minute, or achieve a rotational speed of more than 150,000 rpm. A polygon mirror assembly capable of these facet rates would be structurally difficult to manufacture and operate, and extremely expensive. Due to centrifugal force limitations referenced above, one cannot increase the number of facets per second simply by increasing the number of facets of the same size on the polygon.
Another significant problem confronting prior laser projection systems attempting to produce the high resolution required to compete with film technology is the inadequacy of prior art modulation technology. Each laser beam of the three primary colors must be modulated to produce a different color intensity for each pixel being scanned. For standard television resolution, more than 250,000 modulations must occur for each frame for each color or laser, or a total of 7.5 million modulations per second for 30 frames per second. For high resolution, at 1920×1080 p, more than 2 million modulations must occur for each color or laser to scan each frame, or a total of at least 120 million modulations per second per color for 60 frames per second. For desired non-interlaced (progressive) imagery having even greater resolution, such as 3000×2000 pixels, the rate is above 360 million modulations per second. Current modulation technology is not capable of modulating the laser beams, especially powerful laser beams, at a sufficient rate to enable the generation of the number of discreet pixels required for even film-quality digital resolution.
There are other inadequacies in the existing technology that are not addressed in detail here, but there are significant challenges, including complexity of optics, brightness, resolution, contrast and image stability.